Printed circuit boards (also called printed wire boards or PWBs) are conventionally made of primarily fiberglass fibers, with electrically non-conductive fillers. However there has been increasing interest in making printed circuit boards from aramid fibers since aramid fibers have a number of advantages over glass, or mixtures of aramid and glass fibers are used. For example, duPont Chemical Company uses its own brand of aramid fiber ("THERMOUNT") in the production of printed circuit boards.
The duPont aramid PCBs are made using the conventional liquid laid process for non-woven web production using a foraminous element, such as a wire. In order to effectively make non-woven webs using aramid fibers by the liquid laid process, duPont uses a blend of different length and diameter aramid fibers, some of which may be fibrillated, in an attempt to produce versatile and entirely commercially acceptable printed circuit boards. However there are numerous problems associated with the water laid process of production of aramid non-woven sheets or webs using conventional para aramid fibers (which are "straight").
Conventional aramid printed circuit boards, and layers formed of non-woven webs making up such boards, have a significant number of problems including the inability to randomly disperse the aramid fibers as uniformly as customers would like, and typically the aramid sheets are directional. This directionality creates different coefficients of thermal expansion in the machine direction and the cross-machine direction in the finished product, and in tear characteristics relating to saturating the sheet. Also such boards are difficult to handle and require a significant amount of handling experience by customers, and they have an affinity to absorb moisture so that some customers must bake each roll in an oven to drive off humidity before it can be used. Also great care must be exercised during manufacturing to avoid chain wrinkles, lay flat, and other undesirable features which can be introduced during the forming, calendering, and rewinding processes. Also there is a recognized problem with electrically conductive particulate contamination, which reduces the electrical properties of the web produced.
According to the present invention a printed circuit board layer, the printed circuit board per se, and a method of producing printed circuit boards, are provided which are advantageous compared with conventional aramid-based and glass-based printed circuit boards. According to the invention it is preferred to use the foam process, such as described in U.S. Pat. No. 5,904,809 (the disclosure of which is hereby incorporated by reference herein) and the primary fiber used in the construction of PWBs according to the invention is acrylic fiber, more particularly a high tenacity acrylic fiber such as polyacrylonitrile. According to the invention it has been found that acrylic fibers are highly advantageous in the production of PWBs.
According to another aspect of the present invention, the non-woven web or sheet may be made utilizing the foam process. The foam process is highly efficient in handling fibers like acrylic fibers, allowing the formation of a much more uniform web, and allowing fiber blending to a much better extent than webs produced by the water laid process. Fiber blending may be particularly important in the production of printed circuit board layers containing acrylic fibers. Conventional non-conductive fillers (such as plastic or glass particles) can be incorporated in the foam and are uniformly distributed in the final web produced. Also by using the foam process the density of the fiber-containing webs or sheets produced may be much more closely regulated than when the water laid process is utilized, other fibers such as aramid and glass may be readily incorporated, and the entire formation process is less expensive and more energy efficient.
Utilizing the invention, printed circuit boards, and layers for printed circuit boards, may be produced containing at least 50% acrylic fiber, and preferably about 60-80% straight high tenacity acrylic fiber about 3-12 mm long with a diameter of about 6-15 microns, and about 40-20% fibrillated acrylic fibers (i.e. pulp fibers). Substantially 100% acrylic fiber boards and layers may be produced according to the invention, but there typically will be at least some other non-conductive fibers, like glass fibers, or aramid fibers, or non-conductive fillers, and 0-40% non-conductive organic or inorganic binder.
The web or sheet produced according to the invention is typically densified or compressed (as by using conventional thermal calendering rolls) preferably so that it has a density of between about 0.1-1 grams per cubic centimeter, and a basis weight of between 20-120 grams per square meter. The web or sheet may be binder free, or may comprise about 1%-40% (preferably less than 20%) by weight of a substantially electrically non-conductive organic or inorganic binder.
According to another aspect of the present invention a printed circuit board is provided comprising the following components: A plurality of substantially electrically non-conductive substrate layers. At least one of the layers comprising, prior to pre-preg, a non-woven layer comprising at least 50% by weight acrylic fibers. [Preferably a pre-preg material, impregnates at least some of the layers.] And, electrically conductive circuit elements provided on or between at least one of the substrate layers. Most printed circuit boards are made with between three to six layers, although a significant number of boards are also made using seven to eight layers, and there are also many boards made using nine or more layers. The pre-preg material when used is entirely conventional, and typically is epoxy resin, and the electrically conductive circuit elements are also completely conventional (as is their positioning), typically comprising copper strips, wires, or deposits, or like physical structures of other conductive materials such as silver. Typically the at least one layer containing the acrylic fibers is produced by the foam process (although it may be produced by the water laid process), and may have at least about 90% by weight acrylic fibers prior to pre-preg. Each of the substrate layers may have a density of about 0.1-1 grams per cubic centimeter prior to pre-preg, and the board typically further comprises a plurality of electronic components (such as computer chips, diodes, resistors, etc.) connected to the board substrate, and to the electrically conductive circuit elements, using entirely conventional techniques.
According to another aspect of the present invention, a method of producing a printed circuit board is provided comprising the following: (a) Producing a non-woven sheet or web comprising at least 50% by weight (up to substantially 100%) acrylic fibers, and the balance at least one of substantially electrically non-conductive fibers, filler, and binder. (b) Densifying (e.g. thermal calendering) the sheet or web from (a). (c) Forming a printed circuit board layer using the sheet or web from (b). (d) Combining the layer from (c) with other substantially electrically non-conductive layers, and (e) Providing electrically conductive circuit elements on or between at least one of the layers from (c). There may also be, between (c) and (d), (f) forming a pre-preg from the layer of (c) by impregnating the layer with resin or the like. And, (g) curing the pre-preg of (d)-(f) to produce a printed circuit board.
Procedure (b) is conventional, and typically is accomplished utilizing calendering rollers, and a temperature over 200.degree. C. and a pressure of at least 500 psi. The layering of the sheets or webs to produce the printed circuit board, of (c), and the pre-preg formation of (f), and combining a layer from (c) with other substantially electrically non-conductive layers as in (d), and providing the electrically conductive circuit elements as recited in (e), as well as the securing of (g), are also all conventional. Also there preferably are the further conventional procedures of (h) mechanically acting on the board from (g); and (i) electrically and physically connecting electronic components to the board from (h), and to the circuit elements.
In the implementation of the invention (a) is preferably practiced by the foam process. Also, (a) and (b) are typically practiced to produce a sheet or web having a density of about 0.1-1 grams per cubic centimeter, and (a) is typically practiced using about 40-20% fibrillated acrylic fibers (e.g. about 30%), and about 60-80% straight high tenacity acrylic fibers. Either substantially no binder, or about 1-40% by weight organic or inorganic non-conductive binder, may be used.
The substrates according to the invention, and produced according to the method of the invention, are advantageous compared to the prior art. They have or are:
Far superior resin wet out than aramid papers or woven glass. PA1 Improved fiber consolidation creating less fiber fuzz during resin impregnation. PA1 Easier to cut both in the substrate form, and in the impregnated pre-preg or laminate form. PA1 Easier to laser cut and drill holes because glass absorbs laser energy more than the non-wovens of the invention. PA1 Lower moisture pick up than aramid papers. PA1 Improved dimensional stability. PA1 Improved MD/CD ratio and stability. PA1 Lower weight than glass fiber; and PA1 Good adhesion to impregnating resins typically used in laminate production.
It is the primary object of the present invention to produce acrylic fiber-containing layers, and printed circuit boards formed from one or more of such layers, which have enhanced utility and/or enhanced ease and reduced cost of production, compared to conventional glass and aramid fiber-containing layers or boards. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.